Variable slug-controlled coaxial transformer



Sept. 22, 1970 s. E. PARKER 3,530,410

VARIABLE SLUG-CONTROLLED COAXIAL TRANSFORMER Filed July 1, 1969 i'. 76.I l WP"; 3 I I pg ,L

I NVE NTOR.

3 I 844! E. PAR/(5 A TTOR/VEYS 2 Sheets-Sheet 1 Sept. 22, 1970 s. E.PARKER 3,530,410

VARIABLE SING-CONTROLLED COAXIAL TRANSFORMER Filed July 1, 1969 2Sheets-Sheet 2 REACTANCE X Xi AND X MEASURED AT THREE FREQUENCIES USING|'/2 TURN SO-OHM COAXIAL CABLE WITH AND WITHOT SLUGS 0F BRASS ANDFERRITE FREQ SHIELD REACTANCE (XS) INNER CONDUCTOR (x,) MUTUAL REACTANCE(XM) (NW/SJ NO SLUG BRASS FERRITE NOSLUG BRASS FERRITE NO SLUG BRASSFERRITE |4- 21.3 |7.5 34.6 24.2 22.9 38.6 20.57 l6.27 36.42 2| 32.0 27.458.6 40.2 35.6 59.5 35.67 26.20 64.70 28 44.6 3?.l 85.7 56.9 4'9.| 77.754.05 42.55 l|6.87

FIG. 4

2 x x FREQ k= M R M OHMS M /s) 1/ XS Xi 5o NOSLUG BRASS FERRITE NO SLUGBRASS FERRITE 2| 0.994 0.903 L096 25.447 l5.905 83.722 28 L073 0.997L432 58.428 36.2IO 273.!72

'FIG. 5

2 FREQ u MA (MC/S) SLUG TURNS OUT 3.5 l2B.O 56.7 14 2. 5 27.| |a.4 4

3 5 H) (i) INVENTOR. 2a 2.5 |93.| |oo.7

L5. 57.9 36.2 SAN E. PARKER ATTORNEYS United States Patent O 3,530,410VARIABLE SLUGCONTROLLED COAXIAL TRANSFORMER Sam E. Parker, 3651 LiggettDrive, San Diego, Calif. 92106 Filed July 1, 1969, Ser. No. 838,167 Int.Cl. H03h 7/38; H01q 9/16 US. Cl. 33333 Claims ABSTRACT OF THE DISCLOSURESTATEMENT OF GOVERNMENT INTEREST The invention described herein may bemanufactured and used by or for the Government of the United States ofAmerica for governmental purposes without the payment of any royaltiesthereon or therefor.

BACKGROUND OF THE INVENTION Many techniques are available fortransforming or matching impedance of coupled balanced or unbalancedcircuits. Commonly used lumped-constant devices include twoterminal-pair low-loss LC networks in a form of one or more sections ofL, T or pi configuration. Distributed parameters often enter into thedesign and operation of these lumped-constant networks and play a majorrole in many techniques utilizing forms of transmission lines such asquarter Wave transforming sections, exponential lines, compensatedlines, and line sections in various series and parallel combinations.However, none of these techniques give the convenient adjaustment andthe simplicity of design afforded by the following described compacttransforming structure.

SUMMARY OF THE INVENTION A coaxial cable including a tubular outerconductor enclosing an inner conductor is wound with one or more turnson an insulating tubular form, the two ends of each of the inner andouter conductors being connected, respectively, to the circuits theimpedances are to be matched. A metal slug of either magnetic ornon-magnetic material is adjustable along the axis of the coil and isfound to vary the self and mutual reactances of the windings of thetransformer. The load side of the transformer is turned to show zero orminimum reactance and then the slug is adjusted to match the impedanceof the load to the feed line. Because of interdependence of the self andmutual reactances some tuning may be required after each slugadjustment.

Other objects and features of this invention will become apparent tothose skilled in the art by referring to the preferred embodimentdescribed in the following specification and shown in the accompanyingdrawings in which:

FIG. 1 is a plan view partly in section of one specific coaxialtransformer of this invention;

FIG. 2 shows the coaxial transformer coupling a feed line to a dipoleantenna;

FIG. 3 is a diagram of the equivalent circuit of the transformer of thisinvention;

7 FIGS. 4, 5 and 6 are, respectively, tables of empirical data takenfrom the transformer of this invention.

Patented Sept. 22, 1970 The transformer embodying this invention andshown in FIG. 1 comprises a length of coaxial cable 10 wound on acylindrical tube 12 of low loss dielectric material. The turns may beone or more in number and the pitch of the turns may be fixed withdielectric spacing material, or the turns may be laid one against thenext. The metal member 14, usually called a slug, is of metal and,surprisingly, may be of either magnetic or non-magnetic material. Theslug is adjustable lengthwise within the coil of coaxial cable, thepreferred method of adjustment being screw threads on the slug and inthe insulating cylinder 12. The two ends 1 and 2 of the shield or outerconductor are trimmed back to expose the two ends 3 and 4 of the innerconductor. Each conductor end is suitably capped and connected toscrewdriver terminals on a terminal block for convenience of circuitwiring and switching.

One useful application of the transformer of FIG. 1 is shown in FIG. 2where terminals 1 and 2 of the shield are connected directly to the twohalves 16 and 18 of a dipole antenna and the inner conductor terminals 3and 4 are extended to the feed line 6 of any length and of any impedancecharacteristic. As will be seen the slug 14 can be adjusted to match theimpedance of the antenna load to the impedance of line 6.

FIG. 3 shows a simple schematic diagram of the variable transformer ofFIG. 1. The cable shield or outer conductor forms the inductor L and theinner conductor forms inductor L These inductors are intercoupled withmutual inductance M and coupling capacitance C the latter beingdetermined largely the length and electrical characteristics of thecoaxial cable and very little by the position of the slug. The effectivedistributed capacities C and C are associated respectively, with theinner and outer conductors. It is to be noted that the distributedcapacitances can be represented in various series and parallelconfigurations for different combinations of terminal connections.Energy dissipation is neglected here.

By different combinations of frequencies and numbers of turns in thecoils of coaxial cable commercially known as RG/ 142B/ U, impedancemeasurements show that the insertion of a brass slug causes L L and M todecrease by approximately the same proportion throughout a moderaterange of frequencies. The test frequencies were in the 14 to 28megacycle range. The insertion of a ferrite slug, however, causes the LL and M values to increase by about the same proportion depending on thepermeability of the ferrite material and the range of frequenciesemployed. The table of FIG. 4 shows the changes in L L and M caused byinserting a ferrite slug of medium to high permeability are greater thanthe changes caused by inserting a brass slug of comparable length andcross section.

From the'observations above concerning the increase and decrease ininductances, it should be noted that the coefiicient of coupling k canbe defined as follows:

k=l i L. In terms of the reactances, k can be defined as Mk Jinx.

The coeflicient of coupling is not normally changed appreciably byinserting a magnetic or non-magnetic slug, the coefiicient beingapproximate unity in all cases.

In many antenna matching applications, resonant primary and secondary(load) circuits are often desired. In

0 FIG. 3 a transmission line is coupled to an antenna through thetransformer of this invention. From basic coupled theory under theseresonant conditions the desired mutual reactance X is related to theeffective series primary resistance R and secondary resistance R by theexpression M v pi-t sec where X and X are resonant and are both equal tozero. This equation may be rearranged to show the value of antennaresistance R that matches a 50 ohm feed line Jfn? where R =50 ohms. Inantenna applications these resonant conditions can be obtained in theantenna circuit easier by adjusting the antenna?s length for X or byadding the proper value of reactance in the antenna circuit. Similarly,suitable reactance, such as C, can be added to obtain resonance in theprimary circuit.

The foregoing observations are substantiated by the empirical data inthe table of FIG. 4 where all reactance values are expressed in ohms atthree different frequencies. With no slug, variations of both X and Xexhibit somewhat increasing slopes as a function of frequencyillustrating efi'ects of effective shunt capacities C and C in' FIG. 3.

The measured data in the table of FIG. 4 have been used in computing theapparent coefficients of coupling and antenna input resistance shown inthe table of FIG. 5 that can be matched to a 50 ohm source underresonant conditions. The computed coefficients of coupling which aregreater than 1 are mentioned here to indicate close coupling betweentransformer turns and rather prominent effects of stray parameters.

The values of R shown in FIG. 5 are obtained from computed values of Xfound in FIG. 4. Referring to FIG. 3, reactive measurements were made atterminals 1 and 4 with the coils connected series aiding where terminals2 and 3 were connected together. In addition reactance measurements weremade at each frequency with the coils connected series opposing wherethe measurements were taken at terminals 1 and 3 with terminals 2 and 4connected together. Calling these reactances X,, and X respectively, theapparent mutual reactances were derived from the expression The range ofantenna resistance R shown in the right hand columns of Table 3 of FIG.6 include values of input resistance of a variety of common antennatypes that are commonly operated under resonant conditions. A fewexamples include: a quarter wave thin monopole over good ground, 35ohms; half-wave dipole in free space, 74 ohms; close spaced Yagi-Udaarray, 5-15 ohms; and

ace a two-wire folded dipole in free space, 300 ohms. All values ofinput resistance mentioned above depend upon details of the antennadesign including relative thickness, feedpoint details, heighth aboveground, ground conductivity, and proximity of surrounding objects all ofwhich may produce wide variations in the actual input resistance.

In a very simple compact form the adjustable coaxial transformer of thisinvention affords a rather wide range of resistance transformation.Theuse of a threaded core facilitates precise adjustment. A longdielectrie rod or Wooden pole can reach and adjust the slug at the topof an antenna mast. While brass cores afford advantages ofmachinability, ferrite materials can be used if the ferrite slug iscoated with machinable Teflon or other durable plastics. The advantagesof simplicity and convenience of the transformer of this inventionbecomes especially ap parent when the adjustable transformer is pairedwith the 5 well known delta, gamma or T-match techniques described inradio handbooks. Adjustment of the transformer can cause someundesirable detuning of the antenna circuit as well as of the primarycircuit. By careful selection of cable length so that minimum turns areutilized, the primary and secondary reactances of the coaxialtransformer can be minimized so that, in turn, changes in X and X, fromslug adjustments are also minimized.

What is claimed is: 1 1. In combination:

two high frequency circuits, and a transformer for coupling togethersaid circuits, said circuit having at operating frequencies unequalimpedances looking into the ends of the circuits to be coupled; saidtransformer comprising a coil of coaxial cable, the

ends of the inner conductor of said cable being connected to one of saidcircuits and the ends of the outer tubular conductor of said cable beingconnected to the other of said circuits, and a metal slug disposed onthe axis of said coil and being adjustable along said axis. 2. Thecombination defined in claim 1 further comprising:

reactance elements in each of said high frequency circuits for tuningthe circuits to resonance as said slug is positioned for optimum mutualreactance. 3. In the combination defined in claim 1: one of said highfrequency circuits comprising an antenna, and the other circuitcomprising a twoconductor feed line. 4. In the combination defined inclaim 1 said transformer further comprising:

a tubular form of insulating material upon which is aflixed the turns ofsaid coil, the interior of said form and the exterior of said slughaving matched threads for adjustment of the slug along said axis of thecoil. 5. A device for matching the impedance at high frequencies of twodissimilar circuits, said device comprising: a coaxial cable including atubular outer conductor enclosing and concentric with an innerconcentric conductor, the cable being wound into a coil of apredetermined number of turns, the ends of the inner and outerconductors being adapted for connecting, respectively, to saiddissimilar circuits, and a metal slug being adjustable along to axis ofsaid coil for varying the mutual reactance of said transformer.

References Cited UNITED STATES PATENTS 3,197,723 7/1965 Dortort 336l3,260,977 7/1966 Coltman 336- 60 HERMAN KARL SAALBACH, Primary ExaminerM. NUSSBAUM, Assistant Examiner US. 01. X.R. 336-136, 195; 343-822

